Bert and Ernie Step Out

Last August, I posted a piece about  two high energy (PeV, 10^{15} eV neutrinos that IceCube had observed.  The neutrinos had been announced at Neutrino 2012, the main scientific conference for neutrino enthusiasts.  Now, we (IceCube) has written a paper about the two events, which has been posted to the Cornell preprint server at http://arxiv.org/pdf/1304.5356v1.  The events have not changed much, but we have spent the intervening months refining the reconstructions and evaluating the backgrounds.    We now know the energies much more accurately:  1.04 and 1.14 PeV respectively, with a 15% uncertainty.    1 PeV = 1,000,000,000,000,000 electron volts, compared to 120 electron volts for an electron moving through the wires in your house.

We are also hard at work at follow-up studies, but we're not ready to present these yet.

The two events have been given catchy names, which have been featured in a number of news & blog reports about the events.    The two event displays show the two events.  Each dot is an IceCube optical sensor that observed Cherenkov light from the charged particles produced in the neutrino interaction.  The colors show when the light arrived at the sensor (red = earliest, then yellow, green, blue...), and the size of the circle indicates the number of photons that were observed.

Dark Matter - after 80 years, a mystery endures

Dark matter is one of the enduring mysteries of the universe.  The first signals were observed by Fritz Zwicky back in the 1930's.  Zwicky and others observed that there was not enough visible mass in galaxies to provide the gravitational attraction required to keep them bound as they are.  By 'visible mass,' he meant the mass that could be seen i.e. stars.   He also determined that most of the'missing mass' was more broadly distributed than the visible mass.    Initially, this was thought to be likely due to dust clouds or other non-luminous mass.  However, over the past 80 years, scenarios where the hidden mass consists of 'normal' matter - protons, neutrons and electrons - have been largely ruled out. Among other things, if dark matter were 'normal,' it would have altered the density of heavy (heavier than hydrogen) nuclei produced in the early universe.   We have also observed many different signatures of dark matter, at very different distance scales - individual galaxies, galaxy clusters, and signs from the big bang.

For example, the recent findings from the Planck satellite (right) shows that dark matter is needed to explain the observed fluctuations in the cosmic microwave background radiation.  Further, this dark matter must be 'cold,' that is moving at much less than the speed of light.  In other words, it is relatively heavy; the most popular theories predict masses from 10 to 1,000 times the mass of a proton.   So, by process of elimination, most scientists believe that dark matter must be made of some form of as-yet-unseen particles.  A modification of the rules of gravitation cannot be ruled out, but we have not yet found a satisfactory modification.

There are many efforts to look for dark matter, using diverse techniques.  High-energy physicists are working hard, looking for signs that these particles have been produced in proton-proton collisions at the LHC.

Others physicists are looking for the signature of dark matter interacting in a laboratory detector - "direct detection."  In essence, a dark matter particle will bounce off a target nucleus, causing it to recoil.  The details of the recoil energy spectrum depend on the type of dark matter particle and its mass.  Because the cross-sections are tiny and because very little energy is transferred in these interactions, this is best done using large detectors cooled to cryogenic temperatures, and operated in deep underground laboratories, to avoid the background from cosmic rays. Many different types of dark matter detectors are being pursued, with rather different experimental strategies, involving different types of target material, and different strategies to observe the recoil energy.

Finally, many astrophysical experiments are searching for new signs of dark matter in the cosmos.  These searches rely on the idea that dark matter is likely to be its own antiparticle, so two dark matter particles can collide and annihilate, producing a shower of normal-matter particles, which can then be detected using conventional detectors.    The idea that dark matter is its own antiparticle may seem very strange, but it happens naturally in many theories, such as supersymmetry.   These searches look for dark mater annihilation anywhere that dark matter is expected to cluster.  In other words, anywhere that gravity is strong.  Locally, IceCube has searched for dark matter annihilation in the center of the Sun; a similar search from the center of the Earth is coming soon.   IceCube has also looked for signs of dark matter annihilation in the center and halo of our galaxy, and, coming soon, from nearby spheroidal dwarf galaxies.  The latter are of interest because they are believed to have a very high ratio of dark matter to normal matter.  Of course, for galactic searches, many other particles can be studied.   The Fermi observatory (right) has looked for photons coming from the galactic center and galactic halo.  Last year, there were some reports of photons with an energy of 130 GeV coming from the galaxy, but further instrumental studies are required to know if this is real.

The AMS experiment on  the space station (right)  has recently published a report of an excess of positrons (compared with theoretical expectations), with energies up to 350 GeV.  This excess is of particular interest because the positron fraction appears to increase with energy, while theory predicts that it should decrease.  Of course, the same behavior could be because there is a relatively local source of cosmic-ray positrons.


This is a lot of different approaches to dark matter.   One may wonder if there is a coherent strategy here.  The answer is 'sort-of.'   In the absence of a single clear idea what dark matter is, no single approach is known to work.  So, we are pursuing a large number of different approaches, based on what different scientists (and funding agencies) find attractive.  For the current scope of experiments, the multiple approaches are technically and financially feasible.  If none of the current efforts bear fruit, a larger experiment may be needed; this will require a clear strategy choice.  However, to inject a note of caution, different models of dark matter predict widely varying behaviors (interaction and annihilation cross-sections, etc.), and not all of these models lead to experimentally observable consequences, with current or planned future detectors. 

In short, dark matter is an enduring mystery.  After 80 years of effort in diverse areas, we know a great deal about its affect on the universe.  However, we still don't have any direct evidence for its existence, and we really don't know what it is.

ARIANNA - the 2012 season

Even though this blog has been  quiet, ARIANNA construction has been proceeding.  In November, 2012, a group from the University of California, Irvine, led by Stuart Kleinfelder, visited the site and deployed three radio-detection stations.  These three stations are the first half of the 7-station hexagonal array; the remaining four stations should be deployed next season.

These stations include many improvements beyond the prototypes that were previously deployed.   The electronics have been completely redesigned to use much less power (less than 10 Watts, instead of 30).  The power systems are much beefier, with much larger wind generators, on much higher towers.  They should provide power even though the winds only blow a small fraction of the time.  The towers include a much larger solar panel, which will power the station longer into the Antarctic twilight.  The stations also sports new lithium batteries which replace the old lead-acid gel batteries.  They have a much higher power:weight ratio (key for helicopter transport) and better low-temperature performance.  The stations are able to communicate directly with the internet repeater on Mt. Discovery.

In short, this is the first half of an array that should be able to make a physics measurement, either observing GZK neutrinos produced when ultra-high energy cosmic rays interact with the cosmic  microwave background radiation, or setting a competitive limit on their flux.

High-energy (PeV) neutrinos observed!

At June's "Neutrino 2012" conference, IceCube showed two very interesting neutrino events.  Both were cascades (electromagnetic or hadronic showers), with an apparent energy of about "PeV to 10 PeV."  This is equivalent to the mass energy of over 1,000,000 protons, or about 250 times the energy of one of the protons that are accelerated at the LHC (Large Hadron Collider).   In other words, it is far beyond anything we could imagine producing on Earth.

The events, recorded on August 9, 2011 and on January 3rd, 2012, are shown below.  

The side (top/left grahic) and top (bottom/right) view of the January 3rd event.  Each dot shows one IceCube optical module; the colored spheres show the 312 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons. About 96,000 photons were observed from the event. This is a very small fraction of the total number that were created by the shower.

The side (top/left) and top (bottom/right) view of the  August 9th event. Each dot shows one IceCube optical module; the colored spheres show the 354 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons.About 70,000 photons were observed


In both events, the cascades were far from the detector edges, and there was no sign of any incoming muon, so the events are unlikely to be background.  We are also confident that they are real, and not due to a detector problem of some sort.    Analysis of these events is still on-going (for example, to better determine their energies), but these are clearly far more energetic than the events previously seen by IceCube, and the estimated background from atmospheric neutrinos is about 0.14 event.    That estimate is high because it does not include at least one mitigating factor -if these were created in a roughly downward-going cosmic-ray air shower, then we would have seen evidence of the shower in the IceTop surface detection array.   Also, the 0.14 events is for all types of neutrino interactions with a lot of produced light, while these two events look like either electron neutrinos, or neutral-current (NC)  interactions of any type.  The NC interactions deposit only a fraction of the neutrino energy in the detector, so if these were NC interactions, the neutrinos would have to have been even more energetic.

For those who want more information, Aya Ishihara's talk is posted here.  n.b.  Everything above (including the event plots) is taken directly from Aya's talk.

It is too early to say what these events will mean, but this is a very very interesting development.   Stay tuned.

No Nus is big news

IceCube published a paper in Nature last week, about neutrino production in gamma-ray bursts (GRBs); this marks a significant step forward for the experiment.  Unfortunately, it wasn't an observation - we saw no neutrinos.  It was, however, our first "interesting" upper limit, where "interesting is" defined as heavily constraining current theories of  particle acceleration in GRBs.

The IceCube results disfavor GRBs as the source of ultra-high energy cosmic-rays.   More precisely,  "either the proton density in gamma ray burst fireballs is substantially below the level required to explain the highest energy cosmic rays or the physics in gamma ray burst shocks is significantly different from that included in current models."  There was considerable collaboration discussion about the nuances in this statement; the end result was something that we could all live with.  The nuances were required because current theories of how GRBs accelerate particles are quite primitive, with simplified models of the geometry of the object, the acceleration, and the neutrino production.  So, to rule out a theory, we had to look at not only the central prediction of the theory (the most likely number of neutrinos), but also at how much one can reasonably adjust the parameters in the theory to reduce the predicted number of neutrinos.  We concluded that it is not easy to adjust the parameters of the existing theories enough to reduce the number of neutrinos below our sensitivity level.   It is, of course, likely that theorists will adjust their theories to reduce the neutrino production, but, of course, we will continue to analyze data - this result was based on two years of data with 50% and 75% of the detector complete, respectively.  In the end, though, IceCube is sensitive enough that we should either see a signal, or the required adjustments to the theory will make them seem unattractive.

The paper received considerable media attention, with coverage on MSNBC, the Christian Science Monitor, the BBC, Scientific American, Science News etc.   The Register gets the award for the best language: "Eggheads stumped after killer gamma rays ruled out. Probably"

Unfortunately, most of these writeups were based on IceCube press releases.  You can find the press coverage on google news;  the press releases are available here:

UW Madison
LBNL
DESY

The paper is available from Nature here.  A subscription is required for see this, but the paper is also freely available on the Cornell preprint server, here.

(The photo above is from the SWIFT satellite, showing X-ray emission GRB090212 (which, unfortunately, was not used in this analysis)).

Science in the Theater

"Science in the Theater - Extreme Science" is over. It was an interesting experience, and, although I can't say that the four presentations melded well, they were all both good and interesting, and both the speakers and the audience enjoyed it.

We spoke in the ~ 600 seat Roda theater at Berkeley Repertory Theater. We arrived at 6 pm, for a 7 pm curtain, allowing time for mike and video checks, plus a quick dinner. The sound guy was impressive - I don't see how he could switch our microphones on and off so quickly whenever one of us said anything. It was cool to see the backstage area. My wife Ruth and I had seen Moliere's "A doctor in spite of himself" there only two days earlier. It was interesting to see some of the props up-close, and this certainly contributed to the feeling of being on the 'big stage.'

I brought a prototype Digital Optical Module as a prop. It was a pain to lug around, but made a great prop. As a bonus, I got to park in the Rep loading dock, mere feet from the stage.

The audience started arriving early - a few people were already there when we arrived. It was a good crowd - the lower level of the theater was full, with a sprinkling of people upstairs. It was an interesting mix of ages, with a good number of high school kids, plus some from junior high.

Andrew Minor started off, describing how he uses electron microscopes to explore the effects of extreme environments (such as inside a nuclear reactor) on matter. Then, Caroline Ajo-Franklin talked about her studies of organisms that live in extreme environments (for example, microbes that get their energy by oxidizing metals, instead of using oxygen), and about how we might be able to use the techniques and genes in these organisms to generate energy. Tamas Torok talked about his travels in the former USSR, searching for organisms that live in extreme environments, such as in heavily acidic lakes, volcanic craters, and in Lake Baikal.

Then, I discussed my work, starting with a brief explanation of why we do neutrino astronomy, leading to a 'guided tour' of IceCube and ARIANNA. Then, we had a phone call from the South Pole, with our two IceCube winter-overs, Carlos Pobes and Sven Lidstrom on the other end. I had been a bit nervous about the logistics for this, particularly getting the timing right. In the end, the timing was almost perfect. The audience seemed blown away by the call. One woman said that she couldn't get her head around what a temperature of minus 93 degrees meant; judging by the reaction, this was a common view.

Then we answered questions for an hour. These varied widely in level and in form. A number of people expressed concern, with varying degrees of specificity, about the genetic engineering aspects of the extreme biology.

A video of the event is posted on youtube at http://www.youtube.com/watch?v=zuyaPaFbT3A.
Also, there are photos of the event here.

Many thanks to Carlos, Sven and IceCube outreach coordinator Laurel Norris for their help with the phone call to the South Pole. Also thanks to the LBNL folk who organized this, particularly Jeff Miller and and Dan Krotz, plus videographer Ivan Berry, and also to the folks at Berkeley Rep for providing great logistical support.

"Extreme Science" -- in the theater

On February 27th, I will be one of four participants in a presentation on "Extreme Science" - part of Berkeley Labs "Science in the Theater" series, to be held at Berkeley Repertory's theater. It should be an interesting mixture - two scientists who work in extreme environments (Antarctica and Siberia & other places), and two whose work is extreme in other ways. Each of us will speak for 10-12 minutes, and then answer questions. Previous "Science in the Theater" series have featured scientific quartets with similar interests; it will be interesting to see how this works with four people from diverse backgrounds.

Details (time, location, etc.) are available on the Friends of Berkeley Lab website. There is also a nice promo video on youtube.